For centuries, glass has been the material of choice in a wide variety of applications. Although numerous advanced non-glass materials are currently available, glass is still preferred in many instances due to its relatively low cost, good formability and hardness. However, one disadvantage of glass articles is their lack of mechanical strength. It is also known that surface flaws further reduce the strength of glass, which limits its use in applications where it is exposed to multiple impacts. Therefore, methods for fabricating high strength glasses are most desirable. Accordingly, this lack of mechanical strength has been the subject of much experimental investigation in an effort to develop strengthened glass articles.
A number of methods for strengthening silicate-based glasses have been proposed, some of which have gained widespread usage. These include both physical and chemical strengthening processes. In general, these two types of strengthening techniques create an outer layer or region of glass having high compressive stress overlying an inner tension region. This stress differential between outer and inner regions of the silicate glass articles has been achieved in the past by quenching and by the overlay of low-expansion silicate glass onto a substrate of high expansion silicate glass. In the latter case, as the laminate glass article is cooled, the greater contraction of the glass substrate places the overlay glass in compression. Limited increases in silicate glass strength have also been obtained by chill tempering, which is the most common physical method of silicate glass strengthening. Methods of chemically strengthening silicate glass typically provide an outer compression region or layer which has a different chemical composition then the interior glass region. This may be achieved by forming a composite silicate glass article in which the outer region has a lower coefficient of thermal expansion than the inner region. As the composite is cooled to a temperature below its annealing temperature, this differential in thermal expansion produces a strengthened silicate glass article having an outer compression layer and an inner tension layer.
A low-temperature method for producing an outer or surface compression layer for silicate glass articles is disclosed by S. S. Kistler, "Stresses in Glass Produced By Non-Uniform Exchange of Monovalent Ions," J. Am. Ceram. Soc. 45 [2]59-68 (1962) wherein a chemical method of silicate glass strengthening is disclosed which involves low-temperature ion exchange. Other investigators, notably Nordberg et al. in "Strengthening by Ion Exchange," J. Am. Ceram. Soc., 47 [5]215-219 (1964), describe low-temperature ion exchange strengthening of silicate glasses. Ion exchange treatment of lithia silicate laser glasses is also disclosed in U.S. Pat. No. 3,687,799 entitled "Glass Lasers of Increased Heat Dissipation Capability Made by Ion Exchange Treatment of Laser Glass."
Silicate glasses such as soda-lime-silica, lead-alkali silicate glasses, borosilicate glasses and aluminosilicate glasses exhibit various unique properties and differ from one another in softening points, electrical resistance and the like. In all silicate glasses, silica is the glass former. In essence, an irregular atomic "matrix" or network is provided by silicon and oxygen atoms which are covalently bonded together by highly directional bonds. In some silicate glasses, large metallic cations occupy sites in the network. The properties of silicate glasses are well known. In low-temperature ion exchange or ion "stuffing" of silicate glasses, monovalent ions located in the surface layer or region of a silicate glass article are substituted or replaced by larger ions. usually by a diffusion process. For example, it is known that a silicate glass article can be strengthened by placing the article in a molten alkali bath such as potassium nitrate whereby the potassium ions, which are larger than the sodium ions present in the glass, diffuse into the outer glass layer, replacing sodium ion embedded in the silicon dioxide network. In this method, the ion exchange is carried out below the annealing temperature of the glass so that the stress produced by the presence of the potassium ion sets up an outer compression layer which is not relaxed upon cooling.
While the properties of silicate glasses are understood quite well, silicate glasses are not appropriate for some purposes. More exotic "specialty" glasses have thus emerged in recent years to meet specific needs. For example, it is known that certain non-silicate glasses have special transmittance properties not available in silicate glasses. Some glass compositions transmit X-rays and, of particular interest herein, it is known that germanate glass compositions transmit infrared radiation. Since germanate glasses are more transparent to infrared radiation than silicate glasses, germanate glasses are of considerable interest in infrared technology. Unfortunately, germanate glasses suffer from the inherent drawback of most glasses; they are mechanically weak and are thus easily fractured. This lack of strength severely limits the use of germanate glass articles in many applications where its infrared transmittance properties are highly desirable. One such potential use of germanate glass articles is in connection with infrared homing devices such as missiles where glass domes and windows must be infrared transparent. However, no high strength germanate glass compositions have, to applicant's knowledge, been developed. Attempts at ion exchange strengthening of germanate glass compositions have to date been unsuccessful. Therefore, there exists a long felt need for a strengthened germanate glass article. The present invention provides ion-exchangeable germanate glass compositions and strengthened germanate glass article which satisfy this need.